1. Field of the Invention
The invention relates to polysiloxanes containing carbonate groups and modified with linear polyesters and to their use as additives in coatings.
The compounds of the invention promote leveling, generate antiadhesive properties, and increase the lubricity and scratch resistance of the coatings to which they are added.
Coatings in the sense of the present invention are, in particular, coating materials. These materials can be formulated in a variety of ways and can thus, inter alia, be clearcoats, or coating materials containing pigments or dyes. Their liquid phase can comprise organic solvents and/or water, as is known prior art depending on the binders used. However, the coating materials of the invention need not necessarily include a liquid phase but may also be what are known as powder coating materials. The coating materials may also include the conventional, prior art additives, such as, for example, wetting agents and dispersants, fillers, defoamers, etc., and may cure in accordance with different prior art processes. With particular advantage the compounds of the invention can be used in heat-curable coating materials, in which the polysiloxanes containing carbonate groups and modified with linear polyesters are particularly stable under the curing conditions up to 300.degree. C.
2. Description of the Related Art
It is generally known that polysiloxanes are added to coating materials in order to provide dirt-repelling surfaces or to enhance the lubricity and scratch resistance of such surfaces. Over time, a large number of polysiloxanes and organomodified siloxanes as well have become established in the market. Polyoxyalkylene-modified polysiloxanes in particular have been found suitable as additives for optimizing the surface properties. Polyoxyalkylenes, however, are unstable at temperatures of 150.degree. C. or more under the effect of oxygen. Under such conditions the polymer chain begins to break down, leading in many cases, as a result of the release of insufficiently organomodified polysiloxanes, to defects in the coating composition and/or in particular on the surface of the coating. This can be countered by utilizing the widely known increased thermal stability of polyester-modified siloxanes in comparison with alkylene- or oxyalkylene-modified siloxanes, as described, for example, by I. Yilgor, J. E. McGrath, Adv. Polym. Sci., 86, 1 (1988).
A large number of publications deal with the production and use of silicone-modified polyesters, which generally are branched, for the preparation of binders. Such compounds generally contain reactive groups which allow the overall mixture to undergo a crosslinking and hence curing reaction. In a large number of cases these functional groups are attached to silicon, as taught, for example, by U.S. Pat. No. 5,552,223. Silicone-modified binders of this kind are not comparable with the polysiloxanes containing carbonate groups and modified with linear polyesters, as described in this invention, since they constitute a separate class of binder and are not used as additives which are effective in small amounts.
A similar diversity is found in respect of the production and use of silicone polyesters as elastomers and thermoplastics, as set out, for example, in EP-A-0 146 827. Polysiloxane-polyester block copolymers described therein are optimized with respect to their mechanical properties for subsequent use in shaping plastics and are, likewise, incomparable either analogously or in respect of their activity with the polysiloxanes containing carbonate groups and modified with linear polyesters that are described in this invention.
The preparation of linear polysiloxanes having a blockwise construction and modified with linear polyesters is normally carried out by hydrosilylation in order to obtain a linkage between an alkenyl-functional polyester segment and a hydrosilyl-functional polysiloxane, as set out in DE-A-34 27 208. A particular disadvantage in this case, however, is the complex preparation of an alkenyl-functional polyester segment suitable for the hydrosilylation reaction.
Other processes for preparing linear polysiloxanes having a blockwise construction and modified with linear polyesters do not feature this disadvantage and have been prior art for a considerable time. For example, the reaction of chloro- or dimethylamino-functional polysiloxanes to give polysiloxanes modified with linear polyesters is described in DE-A-20 01 945. It is also possible to start from carboxyl-functional polysiloxanes, as set out in DE-A-35 02 928.
For the preparation of such materials, linear, aminoalkyl-functional polysiloxanes, for example, are first of all reacted with dialkyl dicarboxylates. This leads to the formation of linear .alpha.,.omega.-alkyl carboxylate-functionalized polysiloxanes which are linked via amide bridges and which are then reacted further with linear diols and linear dialkyl dicarboxylates. DE-A-34 27 208 and DE-A-35 35 283 describe how, even starting from hydroxy-functional polysiloxanes, it is possible to prepare linear polysiloxanes having a blockwise construction and modified with linear polyesters, by direct condensation and/or addition reactions with further diol components and carboxylic acids or derivatives thereof.
Very similarly, U.S. Pat. No. 5,488,123 also shows how, starting from linear hydroxyalkylpolysiloxanes, polycondensates of the (AB).sub.n A type, with entirely random distribution, are produced in a condensation reaction at final temperatures of up to 260.degree. C. with the addition of diols and dicarboxylic acids or derivatives thereof.
The above-described processes by means of poly-condensation and/or addition reaction have significant disadvantages which lie in particular in the use in some cases of expensive or difficult-to-obtain polysiloxane derivatives and temperatures of up to 260.degree. C., in association with the pronounced inherent coloration of products obtained in this way. Processes which envisage a reaction at relatively low temperatures and relatively short reaction times readily lead to the formation of nonhomogeneous polymers owing to nonuniformly distributed polysiloxane blocks and polyester blocks. The consequence of this is the formation of siloxane-rich adducts having comparatively low degrees of modification, alongside corresponding polyester-rich adducts. Products so lacking in homogeneity exhibit an inadequate application profile, especially in terms of mixing problems and defects of the coating surface.
This problem can be circumvented by the use of .alpha.,.omega.-hydroxycarboxylic acids and/or their lactones, since in this case, owing to the effective monofunctionality of the polyester unit, the reaction proceeds in a more controllable manner and leads in a targeted fashion to A-B-A polyester-polysiloxane-polyester triblock copolymers. Polycaprolactone-polysiloxane-polycaprolactone-silicone polyesters in particular have been a subject of intensive investigation, as can be seen, for example, in I. Yilgor, J. E. McGrath, Adv. Polym. Sci., 86, 1 (1988). Corresponding products are freely available commercially (for example, Tegomer.RTM. HSi-6420 and HS-6440, Th. Goldschmidt AG, Essen). EP-B-0 208 734, DE-A-34 27 208 and DE-A-35 35 283 also show comprehensively the diverse possibilities of the ring-opening polymerization of lactones on hydroxy-functional polysiloxanes and hydroxy-functional polyoxyalkylenepolysiloxanes. The synthesis starts from linear hydroxyalkylpolysiloxanes, which are reacted with .alpha.,.omega.-hydroxycarboxylic acids and/or their derivatives (for example, .epsilon.-caprolactone). The essential advantage of this synthesis sequence (use of the .alpha.,.omega.-hydroxycarboxylic acids and their derivatives, which permit an extremely strict A-B-A triblock copolymer structure) is at the same time also its fundamental disadvantage, since there is, accordingly and logically, a restriction to only these .alpha.,.omega.-hydroxycarboxylic acids and their derivatives as polyester raw materials. Therefore, in accordance with the commercial availability of these .alpha.,.omega.-hydroxycarboxylic acids and their derivatives, the value of this synthesis strategy is considerably restricted, since only a few raw materials are available and, therefore, there are severe restrictions on the preparation of compounds tailored to specific applications.
In order in particular to pursue a variable synthesis strategy, it is possible, starting from ready-made polysiloxane blocks and polyester blocks, to make use of a linking unit to join the two structural elements. Starting, for example, from polyesters and from polysiloxanes which are in each case hydroxyalkyl-functional, a large number of potential linkage elements are possible. Mention may be made, by way of example, of dicarboxylic acids and their derivatives, bisepoxy compounds and diisocyanates. In the case of the dicarboxylic acids and their derivatives, important disadvantages of these linkage elements are the abovementioned high temperatures and the problematic course of reaction, which leads to nonhomogeneous products. Dicarboxylic dianhydrides, such as pyro-mellitic anhydride, for example, although reacting at low temperatures, nevertheless result in an unwanted acidification of the molecule as a whole. In the case of the bisepoxides as linkage element, the reactivity of the secondary hydroxyl group which is formed in the course of the addition reaction cannot be suppressed, resulting in crosslinked structures. Diisocyanates of low molecular mass are toxicologically objectionable and are complicated to control on the industrial scale. Less toxicologically unacceptable, protected and/or relatively high molecular mass derivatives either require high temperatures again, so that unwanted side reactions (as a result, for example, of allophanate formation) lead to crosslinking, or else the structural effect of the comparatively high molecular mass diisocyanate leads to a marked influencing of the overall properties of the polyester siloxane prepared in this way.
It has surprisingly now been found that polysiloxanes containing carbonate groups and modified with linear polyesters do not have the disadvantages described above and can be prepared in a simple manner by the reaction of carbonates with hydroxy-functional poly-siloxane segments and polyester segments.
In this case, the use of polyester blocks which are widely available commercially enables the polysiloxanes to be modified in an extremely economic and diverse manner. Furthermore, the products obtained in this way also feature particularly low inherent coloration and outstandingly effective technical performance.
The reaction between polysiloxanes and carbonic acid derivatives for the preparation of polycarbonate-polysiloxane block copolymers is prior art. It is described, inter alia, in EP-A-0 764 676 and U.S. Pat. No. 5,608,026. However, polycarbonate-polysiloxane block copolymers prepared in this way all contain carbonate segments joined directly to one another, of the following general structure ##STR1##
in which A is a divalent branched or linear, aliphatic, cycloaliphatic or aromatic organic radical with or without heteroatoms and a is a number which is at least 3.
The polycarbonate segment, which is generally obtained by nucleophilic substitution of diols on phosgene or on cyclic carbonates, is, however, always an essential constituent of the overall molecule in order to achieve a desired pattern of properties.